Fix selecting when opening an image from the download shelf.

After migrating fullPaths/URLs to Entries in the gallery, it was assumed that the 'entries' and 'selectedEntries' arrays contain a common subset of Entries by meaning of same references for the same images.

However, when opening from the shelf, the selected image's Entry reference is not contained in the 'entries' array. Therefore, we have to lookup by urls instead of references. In general, we should never compare Entries by references.

TEST=Tested manually on the download shelf.
BUG=331735

Review URL: https://codereview.chromium.org/124923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243090 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243090